Over the years, engineers have come to recognize that some undesirable emissions can be substantially reduced using particular injection sequences and/or rate shapes at particular engine operating conditions. For instance, engineers have come to recognize that at some engine operating conditions, it is desirable to deliver fuel to the engine cylinder in a so called pilot plus main injection sequence. By injecting a relatively small pilot amount of fuel and then following the same with the main injection event containing the bulk of the fuel for that cylinder, it has been found that the resulting combustion is improved relative to a similar injection quantity injected all at once. In other words, at least one of NOx, unburned hydrocarbons and particulates are reduced when utilizing a pilot plus main injection sequence at certain engine operating conditions.
While it may be known that pilot plus main injection sequences are desirable at certain engine operating conditions, it has proven problematic to consistently and accurately control the relatively small pilot injection. Not only do realistic geometrical tolerances and other factors cause a plurality of otherwise identical fuel injectors to behave somewhat differently when supplied with identical control signals, a given injector may also not produce consistent injection results based upon receiving identical control signals over a plurality of engine cycles. If the injector's behavior deviates too substantially from an expected injection sequence, the goal of lower undesirable emissions from the engine may not be consistently achieved.
The present disclosure is directed to one or more of the problems set forth above.